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Airbnb

Coins in the couch.

Photos: Shop Architects and James Corner Field Operations (Domino); Courtesy of Airbnb (Apartment)

Founded: 2008 Total guests: 8.5 million Guests who stayed in castles this weekend: 123 Lesson: Apartment-swapping can be role-play.

“Airbnb was really born from a math problem,” says co-founder Joe Gebbia. “We quit our jobs to be entrepreneurs, and the landlord raised our rent beyond our means. And so we had a math problem to solve. It just so happened that that coming weekend, a design conference came to San Francisco that just wiped out the hotels in the city. We connected the dots. We had extra space in our apartment—what if we shared that space with them and were able to offset our rent by a little bit? So thus was born the air bed-and-breakfast.” From that desperate, sitcom-ready setup, the San Francisco–based company has grown to connect more than half a million willing hosts and trusting travelers in more than 34,000 cities and 192 countries—from dirt floors to castles to tree houses—thanks to $120 million in venture funding from the likes of Marc Andreessen and Ashton Kutcher.

The First Airbnb Apartment: Click Above for a Tour From the Owner (and Company Founder!)